The use of poliovirion temperature-sensitive (ts) mutants are being used to assess current models of viral morphogenesis. A replica plating technique has been employed successfully to obtain 20 ts mutant clones of poliovirus and these clones are being screened for those which are maturation defective. The role of viral-modified rough endoplasmic reticulum is being explored by isolating and partially purifying smooth and rough membranes from virus-infected cells. The behavior of radio-labelled viral-specific structures and their association with these cellular membranes will be further studied in vitro. Specific attempts are being made at detecting the movement of viral components from one membrane type to another under in vitro conditions.